Drilling by the oil industry in arctic offshore waters, typically from artificial gravel islands constructed to support such drilling activities, creates a need for pipelines from such drilling sites to transport oil and gas to production facilities onshore. Conventional pipeline construction techniques are extremely difficult to implement in the arctic offshore waters, due to the limited ice-free summer work period in arctic waters. Therefore, it is desirable to lay offshore pipelines during the winter time, using unconventional "through-the-ice" pipelaying techniques. Accordingly, pipelaying in the arctic during the winter necessitates cutting a long vertical slot in the ice sheet so that the pipeline may be laid through the ice slot and down to the sea bottom. In addition, a trench must be excavated in the sea bottom to accommodate and protect the pipeline from gouging and scouring effects of moving ice keels, etc. For ordinary trench-excavation techniques, the trench tends to fill in, especially in a sandy soil, and typically five to ten times as much soil must be excavated as in a vertical sidewall ditch. After the trench is dug, yet another operation is required to construct and lower the pipeline into the trench, meanwhile keeping the ice slot open and keeping the trench from filling in. All of these operations require considerable coordination, and failure in any one operation substantially increases construction time and cost.
Applicant is not aware of any prior art reference which, in his opinion as one skilled in the pipeline art, would anticipate or render obvious the novel method and apparatus of the present invention; however, for purposes of fully developing the background of the invention and establishing the state of the requisite art, the following U.S. patents are set forth: U.S. Pat. Nos. 3,950,955; 4,091,629; 4,409,747; 4,043,135.